A Rogowski coil sensor 12 is shown in FIG. 1. The flexible coil 12 can be clamped around a current carrying conductor 10 without interrupting the conductor circuit, is linear over all practical current ranges, and forms therefore an attractive current sensor solution in the AC power industry. With the AC line centered in the coil 12, the produced voltage V is:
      V          i      ⁢                          ⁢      n        =                              μ          0                ⁢        n        ⁢                                  ⁢        A                    2        ⁢                                  ⁢        π        ⁢                                  ⁢        r              ⁢                  ⅆ        I                    ⅆ        t              ⁢                  ⁢    A    ⁢          <<              r        2            with μ0 the permeability of free space, n the number of turns of the coil, A the coil cross section, r the radius as shown in FIG. 1, and I the line current. The voltage V is proportional to the differentiated line current, and signals V and I will thus show a 90 degree phase difference. A signal V in phase with current I will require integration. Integration will also lead to the correct current profile if higher harmonics are involved.
FIG. 1 shows an example of an integrator 14 coupled to coil 12 at terminal 16 with basic harmonic transfer function:
            V      out              V              i        ⁢                                  ⁢        n              =                    R        1                    R        3              ⁢          1              1        +                  j          ⁢                                          ⁢          ω          ⁢                                          ⁢                      R            1                    ⁢                      C            1                              Integrator 14 with the shown component values is an inadequate solution when accurate phase information is required. The ωRC product at 60 Hz is only about 1.2 resulting in integrator 14 having a substantial undesirable phase shift at the line frequency. The desired phase shift of 90 degrees is obtained with removing R1 so that the transfer function obtains the pure integrator form:
            V      out              V              i        ⁢                                  ⁢        n              =      1          j      ⁢                          ⁢      ω      ⁢                          ⁢              R        3            ⁢              C        1            
The practical problem with this transfer function is that the amplification at DC becomes infinite. As a result, the output can contain an undefined DC level that in essence represents the integration constant leaving the feedback capacitor C1 DC charged. Scholastic indefinite integral calculus exercises ignore the integration constant, i.e. make it zero, and the challenge is now to extend this convenience to the present practical case. One remedy is to place a transistor (MOSFET) across the feedback capacitor C1 to occasionally discharge it so that the OPAMP 15 DC output becomes redefined. This approach requires timing circuitry to discharge infrequently but still regularly at a preferred moment. When well implemented with near perfect MOSFETs this approach can provide for the remedy.
Accordingly, a need exists for an improved integrator for Rogowski coils or other current sensors.